June 2013

Installing Caissons and Conducting Water Column Studies

Wednesday, August 07, 2013

A strong plume of methane-rich bubbles issue from the seafloor at the Summit of Southern Hydrate Ridge. This area collapsed sometime between 2010 and 2011, probably due to methane release from the subsurface. VISIONS '13, Leg 4

Photo credit: NSF-OOI/UW/CSSF

The remotely operated vehicle ROPOS used a snap hook connected to a line on its underbelly, hooked into the frame of the CTD for recovery to the surface. The CTD wire failed as the sensor-sampling package was being recovered onto the deck of the Thompon. Luckily, after its >9000 free descent to the seafloor, it landed upright in the soft sediment. Photo credit: NSF-OOI/UW/CSSF.

Yesterday, after transiting west ~ 5 hours from Newport Oregon to Southern Hydrate Ridge, we conducted an ~ 8 hr dive (R1627) with the ROV ROPOS to install a caisson adjacent to where we will be installing three secondary junction boxes next year and a variety of cabled instruments (e.g. mass spectrometer for gas chemistry, seismometers, digital still camera, fluid samplers and fluid flow meters etc). This caisson serves an important function in the science of earthquake detection (local and basin scale). Next year a broadband seismometer will be deployed inside the caisson and covered in silica beads, and connected to a junction box via an extension cable. The silica beads couple the seismometer to the seafloor, effectively dampening out other acoustic noises, resulting in 'clean' seismic signals and precise locations and magnitudes of earthquakes to be calculated.

The caisson was taken down with the tool sled beneath ROPOS and gently deployed on the seafloor (water depth ~ 780 m). ROPOS moved the caisson into place and over a 4 hour period used the "zip pump" - a high powered vacuum pump - to extract the sediment from inside the caisson. Alternating with this procedure, ROPOS sat on the caisson with the tool sled to push it into the sediment. Once it was nearly flush with the surrounding heavily sedimented seafloor, a lid was placed over the caisson to keep 'critters' from making their home in it over the coming year.

Following the successful caisson installation, UW Oceanography undergraduate student Brendan Philip, used ROPOS to image an amazing methane bubble plume issuing from a methane seep site ~100 m away from the seismometer, followed by collection of methane-rich fluids through the water column using niskin sample bottles on ROPOS. Brendan is interested in quantifying the amount of methane that may reach into the atmosphere in seep environments, because of its importance as a greenhouse gas. He will measure the methane concentration of the samples when we are back in Seattle. ROPOS was then retrieved on deck.

The early morning hours of today (August 7) proved to be more exciting than we wanted. The R/V Thompson transited ~ 1 hr to the base of the subduction zone, near Primary Node PN1A and conducted a full water column CTD. A the CTD was brought to the surface the wire parted and the CTD with 24 niskin bottles fell >9000 feet to the seafloor. The Thompson and ROPOS crew, and Chief Scientist met to discuss the recovery operation, resulting in Dive 1628 to recover the CTD beneath the ROV ROPOS. ROPOS descended to the seafloor and within 10 minutes found the CTD (standing upright!) and within little time snapped a hook onto the frame that secured the CTD to the underbelly of ROPOS. ROPOS ascended to the surface and the port crane was used to secure the CTD, the line connecting the CTD to ROPOS was detached, and the crane moved the CTD back to its normal position on the starboard side of the ship. Early inspection shows that the CTD instruments are likely ok - whew!